How to identify missing collections by Python?
Read from the original
For example sake have library &quot;xxxx.blend&quot;, in it I have renamed a collection from &quot;OLD&quot; to &quot;NEW&quot;, the previously linked collection &quot;OLD&quot; is in my blend but broken.
All the collection names in the library.
&gt;&gt;&gt; lib = D.libraries['xxxx.blend']
&gt;&gt;&gt; with D.libraries.load(lib.filepath) as (data_from, data_to):
...     data_from.collections[:]
...     
['NEW']

All collection names in my blend linked from lib
&gt;&gt;&gt; [c.name for c in D.collections if c.library is lib]
['OLD']

If it's the same collection, renaming and reloading does the trick.
&gt;&gt;&gt; D.collections['OLD'].name = 'NEW'
&gt;&gt;&gt; lib.reload()

.. or get rid of it
&gt;&gt;&gt; D.collections.remove(D.collections['OLD'])

Careful note re above that linked collections can have the same name as an existing collection. So to use keying and be sure it is the one desired, add the filepath of the library
&gt;&gt;&gt; D.collections.keys()
['NEW', 'NEW']

&gt;&gt;&gt; D.collections['NEW', lib.filepath]
bpy.data.collections['NEW']

&gt;&gt;&gt; D.collections['NEW', lib.filepath].library is lib
True

Use sets
For a more robust solution recommend using sets. Using example above compare the names in library
set(data_from.collections)

against
set(c.name for c in D.collections if c.library is lib)